In a number of refining processes, for example hydrodesulfurization, reforming, hydrocracking and others, a liquid or gas feed stream is contacted with a bed of solid particles which catalyze the desired reaction. Such reactors are often used also in chemical processes. Since the catalyst particles used are often quite small, to obtain the most effect of relatively expensive catalyst materials and to insure good contacting, the passageways through the catalyst bed are relatively narrow and are subject to plugging by extraneous solid materials which may enter the bed or be formed therein. It has been found in many processes that corrosion products, scale, and other solid materials accumulate in the catalyst bed and are not washed through. Over a period of time, these particles build up and block flow which causes channeling through the catalyst bed, rendering it less effective and increasing pressure drop substantially. After a period of operation the catalyst bed becomes so obstructed by the accumulation of solid particles that the operation must be stopped and the reactor vessel opened for removal of the accumulation of solid particles, often accompanied by replacement of a significant portion of the catalyst bed. In some processes, the on-stream period between cleanings may only be several months, substantially reducing the service factor and increasing the cost of operation.
That this has been a significant problem to the industry is indicated by the substantial number of patents which have issued covering distributors, scale traps and combinations thereof. A partial list of patents which represent prior art in this field are the following: U.S. Pat. Nos. 2,961,304, 3,006,740, 3,112,256, 3,146,189, 3,255,159, 3,431,084, 3,598,539, 3,607,000.
While it might be thought that a fine screen covering the entire reactor cross-section could be used to intercept fine particles coming into the reactor, this is not practical for both structural and functional reasons. A screen which is small enough to catch all of the very fine particles would be subject to very quick blinding, causing an even earlier shut-down that when operating with no screen at all. It has been found, however, that using a mesh screen on baskets which cover a large portion of the bed that the larger particles are collected, but smaller ones pass on through into the bed where they collect and in time will cause the difficulties previously mentioned, forcing a shut-down for cleaning. When this happens the screen and its supporting structure must be removed from the bed through relatively small manholes, usually having openings of only 18-24 inches. The scale traps and screens must be removed in order to clean the bed and to replace the upper layers of catalyst. Once this is accomplished, then the traps must be cleaned and replaced in order to continue operation. U.S. Pat. Nos. 3,006,740 and 3,598,539 represent typical prior art scale traps which have been used with reasonable success. It will be appreciated, however, that such structures are complex and are thus not only costly to install but difficult to remove and to clean and replace at a time when speed is essential to minimizing down time.